1. Field of the Invention
The present invention relates to a field effect semiconductor device and a manufacturing method therefor and, more particularly to a field effect semiconductor device immune to hot carrier effects.
2. Description of the Prior Art
Recently, the advance of the technique of the large scale integration in the semiconductor field has been very significant. Along with demand for increased packing densities and high operation speeds, the dimmension of constituent elements become extremely small.
Because driving voltages required of semiconductor devices are not necessarily decreased in proportion to the element dimmension, several problems due to the fine structure appear to be solved from the view point of reliability. Hot carrier effect is one the difficult problems which dictate the limitation of the reliabilty in submicron-order devices. The average kinetic energy of carriers moving in a semiconductor is assumed to be 3kT/2 using Boltzman constant k and the absolute temperature T. These carriers receive energy from an electric field in which they are moving. The accepted excess energy, however, is usually dissipated into interaction with the lattice of the semiconductor to achieve thermal equilibrium. When the surrounding electric field substantially increases, hot carriers appear which have a larger average energy level than 3kT/2 because accelation by the electric field overcomes alleviation by interaction with the lattice.
Such hot carriers appear in a position to which electric field is concentrated, for example, in the vicinity of a drain and gate insulating film of a field effect semiconductor device. Those carrier with enough energy to overcome the potential barriers of the gate insulating film are emitted into and trapped by the overlying gate oxide film. The trapped carriers form space charge and vary the characteristics of the device such as the threshold voltage or gm (mutual inductance), resulting in low reliability.